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Atmospheric Effect on the Ground-Based Measurements of Broadband Surface Albedo” by T

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Atmospheric Effect on the Ground-Based Measurements of Broadband Surface Albedo” by T Atmos. Meas. Tech. Discuss., 5, C140–C152, 2012 Atmospheric www.atmos-meas-tech-discuss.net/5/C140/2012/ Measurement © Author(s) 2012. This work is distributed under Techniques the Creative Commons Attribute 3.0 License. Discussions Interactive comment on “Atmospheric effect on the ground-based measurements of broadband surface albedo” by T. Manninen et al. Anonymous Referee #1 Received and published: 5 March 2012 1 General remarks The manuscript provides a parametrization of the relation between measured surface albedo and black-sky surface albedo. The parametrization depends on aerosol opti- cal thickness (AOD), solar zenith angle and the incoming direct and diffuse radiation and can be used for an atmospheric correction of surface albedo measurements. The parametrization was developed with radiative transfer simulations by using data bases of black-sky surface albedo and AOD. The application of the atmospheric correction to surface albedo measurements obtained from a BSRN station in Cabau showed dif- ferences of about 5% between measured and black-sky albedo while the simulations showed maximum effects of up to 20%. C140 The idea to have an simple and robust parametrization for the atmospheric correction of surface albedo measurements is highly welcome and worth to be published. However, the approach presented by the authors suffers of several systematic and methodical errors which have to be reassessed in detail before publishing the manuscript. The accuracy of the atmospheric correction presented in the manuscript is quite limited, which might be caused by some of the systematic errors. If the accuracy can not be improved, I doubt that this method is sufficient to replace an ordinary atmospheric correction which fits the simulation to the measurements. Below, I compiled a list of comments which have to be considered in a revised version of the paper. When writing the comments I sometimes did not consider, which in direction the revised paper might be changed. This may result in some contradictory statements. I am sure the authors will know how to weight in such cases. 2 Major comments Normalization of surface albedo: The authors normalized the measured surface albedo to a solar zenith angle of 60◦ using equation 1. This equation only hold for the black-sky albedo as stated in the introduction by the authors itself. The measured surface albedo is the blue sky albedo and affected by the illumination from both direct and diffuse solar radiation. The partition between direct and diffuse radiation strongly depends on solar zenith angle itself. This means that the surface albedo changes with solar zenith angle for two reasons. a) the black-sky albedo changes, b) the diffuse fraction changes. This normalization may hide some of the atmospheric effects and may explain some of the deviations between corrected albedo and black-sky albedo. Simulations: The authors used the radiative transfer model SPCTRAL2 to calculate the diffuse irradiance Fdiff . The direct irradiance Fdir is calculated by the law of Lamber- Beer. I do not understand why both Fdiff and Fdir are calculated with different methods. C141 SPCTRAL2 also provides Fdir. So there is no reason to do it yourself. Further the calculation of Fdir is fundamentally wrong. In Eq. 3 only the aerosol optical thickness is used while the atmosphere consists also of molecules. The Rayleigh optical thickness has to be included here as well. See the description of SPCTRAL2 (Bird and Riordan, 1986) or just use the results of the model. AOD: For the simulations a range of suitable AOD is derived from AERONET mea- surements at Cabau. From this data set, the parametrization is derived. What about AOD values which are not covered in the 7 month period? I suggest not to focus on the measured AOD in this case. It would be much more appropriate to use a distinct grid of AOD for the simulations. Vary AOD and the Angström parameter systematically within a certain range and run the model. The results can be interpreted much better than the data shown in the manuscript. E.g. in Figure 4 not all categories of solar zenith angles have the same range of AOD. How to interpret the different length of the horizontal bars, if the AOD range is different for each solar zenith angle? How the parametrization will work for AOD values which are not covered by the simulation? To characterize the spectral behavior of AOD, the Angström equation is often used as mentioned by the authors in section 3. Angström exponents have been calculated but never be shown or used. In order to obtain a parametrization which has a more general character, I suggest to express AOD by the Angström exponent and the AOD at the reference wavelength throughout the manuscript. Parametrization: The form of the parametrization does not suit the intention of the study providing a simple parametrization from which surface measurements can be corrected without big effort. There are redundant parameters in the equation. I do not understand, why the diffuse and direct irradiance are used as parameter. Both are calculated from the SPCTRAL2 model, as I understand, and they are functions of solar zenith angle and AOD. This means, a parametrization on solar zenith angle, surface albedo and AOD would be sufficient, Fdiff and Fdir have not to be calculated additionally. C142 Alternatively, for the case, that Fdiff and Fdir are measured at a radiation station, but AOD is not, I suggest to derive a parametrization on measured albedo, Fdiff and Fdir, without AOD. This would be a simple and helpful parametrization. Instead of the AOD at two different wavelength, I suggest to use the parameters of the Angström equation. Use of BSRN-Data: Further I do not understand why no single measurement of Fdir and Fdiff is included in the study. The data which was used in the study comes from a BSRN station where Fdir and Fdiff are measured. At least show that your model results agree with Fdir and Fdiff from the BSRN station. I know the comparison may lack due to strong forward-scattering but with regard to the parametrization it is worth to include measured Fdir and Fdiff . Atmospheric correction: The atmospheric correction using the proposed parametrization does not obtain good results. As shown in Figure 7, the difference between measured and black-sky albedo is reduced only by about 50 %. This is sur- prising as in Figure 7 simulations have been used as input for the atmospheric cor- rection. As the parametrization is based on the same simulations, I would assume a perfect agreement between corrected and black-sky albedo if the parametrization is good. This seams to be not the case. Reasons might be diverse. One might be the above mentioned errors in the method itself. To show that the parametrization is a useful alternative to an complete atmospheric correction, both methods have to be compared in the study. I suggest to apply an atmospheric correction using model simulations by fitting the model to the measured parameters (uncorrected albedo and AOD). I suppose for the problem presented here using irradiance only, such an atmospheric corrections is not time consuming. The results will show, if the method using the parametrization equation is needed at all. Satellite Data: The authors motivate their work by claiming that their method will help to validate satellite surface albedo estimates. Why this comparison is not done? C143 Wording: The nomenclature of the different measured and simulated albedos is to- tally confusing. Some examples: "simulated pyranometer measurement estimate of a surface albedo", "regression based atmospherically corrected value" "simulated pyra- nometer measured broadband surface albedo". The naming of the different albedo has to be consistent otherwise the reader can not follow. The best way is to define the albedo once and than use the symbol of the quantity only. Further, I strongly recommend the paper to be proofread by a native English speaker for grammar and punctation. Figures: The labeling of most figures is to small. Different data points are not capable of being differentiated in some figures. 3 Minor comments P386, 3: Specify in which way the measurements are affected by the atmospheric con- ditions. What do you mean with "atmospheric conditions". Mention that you propose an atmospheric correction. P387, 10: Ground based measurement with goniometer using an artificial radiation source can be used to derived BRDF and thus the black sky albedo. There are several publications on such kind of measurement, e.g. "Dumont, M., Brissaud, O., Picard, G., Schmitt, B., Gallet, J. C., and Arnaud, Y.: High-accuracy measurements of snow Bidirectional Reflectance Distribution Function at visible and NIR wavelengths - com- parison with modelling results, Atmos. Chem. Phys., 10, 2507–2520, 2010.", "von Schoenermark, M., Geiger, B., and Roeser, H.-P., eds.: Reflection Properties of Veg- etation and SoilWith a BRDF-Data base, vol. 1, Wissenschaft und Technik Verlag, 2004." P387, 12: Satellites do not directly measure blue-sky albedo. Only radiances are C144 measured which are used in atmospheric correction schemes to derive the surface black sky albedo, blue sky albedo and white sky albedo. P387, 20: I do not understand. What changes for the analysis of the satellite measure- ments? Here you still have to consider both effects. P387, 28: Change "radiation flux density" into "radiant flux density" P388, 9: There must be a plenty of studies investigating the diffuse and direct fraction of solar radiation. This is a basic measurement at any meteorological site since many years and also used for retrievals of atmospheric properties. A quick web search lead me to the following publications, randomly chosen: "Continental aerosol properties inferred from measurements of direct and diffuse solar irradiance, Marsden et al, JGR, 2005", "The diffuse-to-global and diffuse-to-direct-beam spectral irradiance ratios as turbidity indexes in an urban environment,Kaskaoutis and Kambezidis, JASTP, 2009", "Coupling diffuse sky radiation and surface albedo, Pinty et al, JAS, 2005".
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